How long is the copper wire wound on the coil of the electric bell, if its resistance is 0.68 Ohm

How long is the copper wire wound on the coil of the electric bell, if its resistance is 0.68 Ohm, and the cross-sectional area is 0.35 mm2

1. Let’s translate all the data from the condition into the SI system:
Ssection = 0.35 mm² = 3.5 * 10-5 m²
2. Let’s write down the expression to determine the resistivity of the wire:
ρsp = R * Ssech / l, where R is the resistance of the wire, l is the length of the wire, Ssech is the cross-sectional area of the wire.
3. Let us express from this the length of the wire l:
l = R * Ssec / ρsp
ρsp of copper is determined according to the reference book 1.7 * 10-8 Ohm * m
4. Substituting the numerical values, we have:
l = R * Ssection / ρsp = 0.68 * 3.5 * 10-5 / 1.7 * 10-8 = 1400 m
Answer: 1400 meters of copper wire are wound on the spool.